Electron gun and cathode heater assembly therefor



1964 G. K. MERDINIAN 3,132,275

ELECTRON GUN AND CATHODE HEATER ASSEMBLY THEREFOR Filed May 31, 1960INVENTOR. GEORGE K. MERD/NIAN .4 Z BY Milk/92%.

ATTORNEYS United States Patent 3,132,275 ELECTRON GUN AND CATHODE HEATERASSEMBLY THEREFOR George K. Merdinian, Daly City, (Zalif, assignor toEitel- McCullough, Inc., San Carlos, Calif, a corporation of CaliforniaFiled May 31, 1960, Ser. No. 32,755 11 Claims. (Cl. 313-82) -Thisinvention relates to electron guns and particularly to thermionicelectron guns for use in beam tubes.

One of the industries most affected by advances in technology is theelectronics industry. In this industry manufacturers have had to searchdiligently for innovations in product design which would not onlyimprove the operation of their product, but which would also result inthe lessening of the cost to manufacture the product. The industry hasbecome so competitive that a few cents saved on the cost ofmanufacturing a component can mean the difference between the acceptanceor rejection of bids submitted in Government contracts for the supply oflarge quantities of electron components.

Some of the considerations which face the electronics manufacturers arereduction in weight of their components due to the likelihood ofinclusion of their electronic components in space vehicles and missilesWhere the Weight factor is extremely important. Also critical in thisapplication is the ability of the electronic component to Withstandextremes in acceleration, shock, vibration, and temperature. Sincereliability under these adverse conditions is a prime necessity, it hasbecome the practice of electronic manufacturers to buy only the bestquality materials available regardless of material costs. Since thesequality materials are expensive, one method of effecting a saving inmaterial costs is to use a minimum number of parts arranged for maximumefficiency. It is therefore an object of the invention to provide athermionic electron gun in which the elements of the gun are arranged tocooperate in a manner providing a rigid but light construction havingthe desired thermal characteristics for maximum efiiciency.

Because reliability is a prime consideration, and because many of theparts used in electronic components are extremely small, mruch handassembly has heretofore had to be done in the manufacture of electrontubes. The labor cost for the assembly of electron tubes is therefore nosmall factor in the ultimate cost to the consumer of these devices. Itis therefore another object of the invention to provide an electron gunin which the elements lend themselves to being mass produced, andassembled using assembly line techniques which minimize laborexpenditures.

In environments where electron tubes are accessible, it is desirablethat the tube be designedtppermit replacement of defective parts. Thisis particularly True with regard to those parts of an electron gun, forinstance, which are noted for their short life, such as the heater andcathode of the electron gun. It is therefore a still further object ofthe invention to provide an electron gun in which the cathode andcathode heater coil may be easily removed and replaced and the envelopereevacuated.

Because electron gun elements are enclosed in an evacuated envelope, andbecause they operate at very high temperatures, it is necessary thatmaterials having a low vapor pressure be used. These materials, however,are expensive, and it is therefore another object of the invention toutilize materials having a somewhat higher vapor pressure and thereforelower cost, arranged in the vacuum envelope in a manner to precludevaporization.

The invention possesses other objects and features of advantage, some ofwhich, with the foregoing, will become apparent from the followingdescription and drawings which disclose one embodiment of the invention.It is to be understood that the invention is not limited to theembodiment described and chosen for illustration, as variant embodimentsmay be adopted within the scope of the appended claims.

Briefly described, the invention comprises a thermionic electron gun foruse in an evacuated beam tube. The tube envelope hermetically enclosesthe gun elements, the main supporting element of the gun comprising abase plate having a cylindrical envelope portion integrally brazedthereto and extending away therefrom. The base plate also supports afocusing electrode, together with a heater coil and a cathode having aconcave emitting surface adapted to project a beam of electrons axiallythrough the envelope. A suitable radio frequency interaction structureis provided hermetically interposed between the electron gun and acollector forming the end of the envelope remote from the gun. Means areprovided for rigidly supporting the cathode and cathode heater coil inan efiicient heat exchanging relationship within the cylindricalenvelope portion attached to the base plate. Terminals supported on thebase plate exterior of the envelope are provided with conductive meansextending into the envelope and electrically connected to the focuselectrode and cathode-heater assembly.

Referring to the drawings:

FIGURE 1 is an elevational view illustrating a beam tube with theelectron gun attached thereto.

FIGURE 2 is a vertical half-sectional view of the elec tron gun of myinvention.

FIGURE 1 is shown greatly reduced in scale with a portion omitted toreduce its length. FIGURE 2 is shown approximately actual size.

In more specific terms, and referring to the drawings, the electron gundesignated generally by the numeral 2 is integrally united in acomposite beam tube structure including RF interaction section 3 and acollector 4. The gun comprises a base plate 5, fabricated preferablyfrom copper, and provided adjacent its outer periphery on one flat sidethereof with an annular groove 6. On the same side of the base plate areprovided a plurality of circumferentially spaced recesses each adaptedto receive a short cylindrical copper post 7. The bottom end of eachpost is integrally brazed in a recess, while the upper end portion ofthe post is centrally bored and tapped to provide a means of attachingrelated structure. On the opposite side of the base plate, a pluralityof recesses 8 are provided at circumferentially spaced intervals, andprovide a means of seating copper posts 9 which extend perpendicularlyto the base plate and are provided with a central bore 1 2 useful in theattachment of associated circuit elements (not shown) to the base plate.Apertur es 13 formed in the base plate provide a means ofpassirmductorsathrough the base plate for connection to the electrodeelementsvvithinvthenenldgpe.

Associated with each of the peripheral grooves 6 inthe base plate is acylindrical ceramic wall section 14 sealed at one end to the base plateby means of an annular sealing ring 16 hermetically brazed adjacent itsinner periphery to the metalized end of the ceramic cylinder 14. Aportion of the sealing ring 16 adjacent its outer periphery isintegrally brazed or Heliarc welded to the outer end portion of acomplementary cylindrical flange 17. The other end portion of thecylindrical flange 17 is brazed or otherwise integrally and hermeticallyjoined to the base plate adjacent its outer periphery, this union beingconveniently made by extending the flange into the groove 6 where it isjoined to one of the sides thereof. An annular ceramic backing ring 18brazed adjacent the inner peripheral portion of the sealing ring 16 onthe side thereof opposite the cylindrical ceramic section 14, preventsthe build-up of stresses in the hermetic union, occasioned byfluctuations of temperature, from rupturing the union between thesealing flange l5 and the cylindrical section 14. The bottom surface 19of the backing ring abuts the bottom of the groove 6 in the base plateand provides for relative sliding movement between the backing ring andthe base plate due to variations in temperature. It will thus beapparent that one end of the cylindrical ceramic section 14- isintegrally and hermetically brazed adjacent the outer periphery of thebase plate. The nature of this union will also make apparent that theintegrally joined edges-21 of sealing flanges 16 and 17 may be ground orcut off in order to open the envelope should this become necessary. Forthis reason it is preferred that the edges of the flanges be Heliarcwelded rather than brazed.

Rigidly and conductively mounted on the base plate, and coaxiallyarranged with respect to the cylindrical dielectric wall section 34, isa cathode and cathode heater assembly. The cathode and cathode heaterassembly is detachably but rigidly mounted on the base plate by means ofscrews 22 extending through the radially extending flange 23 of thecathode and heater assembly support ring which includes cylindricallyextending flange 24. Because of its rigidity, the assembly ring ispreferably fabricated from Kovar, which is very strong and particularlyadapted for use in environments such as this where relatively largediameters and thin cross sections are used. Brazed or spot welded aboutthe cyliudrically extending flange 24 is the lower end portion of acylindrical molybdenum support cylinder 26. The upper end portion of thecylindrical support 26 is brazed or spot welded to the lower end portionof a cathode assembly support cylinder 27, preferably fabricated fromnickel and provided at its upper end with a reduced crosssection portion28 spot welded to the upper end portion of an apertured nickel cathodesupport cylinder 29. The cathode support cylinder 29 is integrallybrazed or spot welded adjacent its upper end to the outer periphery ofthe cathode 31, which is provided with an emissive coating on itsconcave surface 32, and provided also with a central aperture 33 forpurposes which will subsequently be described.

The nickel cathode support cylinder 29 extends downwardly from thecathode in coaxial arrangement with the support cylinder 27. The lowerend of the cathode support cylinder 29 is provided with an inwardly orradially extending flange 34 adapted to underlie and to be spot weldedor otherwise integrally secured to the outer peripheral edge portion ofdish-shaped molybdenum heat reflector 36. Apertures 63A in the supportcylinder 29 prevent the entrapment of air molecules in the chamber soformed. As shown in FIGURE 2, a second dish-shaped molybdenum heatreflector 37 lies within the chamber and superimposed above the heatreflector 36, but axially do spaced therefrom a distance determined bythe height of dimples 38. The molybdenum heat reflector 37 thereforefunctions both to reflect heat toward the cathode area, and alsofunctions as a spacer between the molybdenum heat reflector 36 and stillanother axially spaced heat reflector 39. The dished heat reflector 39is provided adjacent its outer periphery with portions or tabs 41 struckfrom the material of the shield and projecting axially therefrom toprovide spacing elements to space we reflector 39 from yet anotherdished molybdenum shield at which closely underlies the windings of aheater coil 43. The heater coil is provided with a non-conductivecoating as is well known in the art, and is pressed into conformationwith the concave surface of the heater shield 42 by means of anapertured molybdenum coil plate 44. The heater windings are preferablyfabricated from tungsten wire, and the coil plate 44 is supportedadjacent its central portion by an axially extending molybdenum pillar46. The pillar 46 is preferably fabricated from tubular stock, and isprovided adjacent one end with integral tabs 47 struck from the tube andspot welded to the concave surface of plate 4-4. At its other end thepillar or hollow receptacle 46 is abutted against the bottom surface ofthe cathode about the central aperture therein. In order that this unionmay be free from vibration, the edge of the aperture in the cathode isconveniently rab beted, and the upper end of the pillar fits snugly intothe rabbet, with the inner diameter of the aperture being substantiallyequal to the inner diameter of the hollow pillar. Reverse ionbombardment which would normally impinge on the cathode is thuschanneled into the pillar where the ions impinge and are trapped on thecentral portion of plate 44 and the interior of the hollow pillar 46.

Confined between the outer peripheral portion of the plate 44 and thebottom convex surface of the cathode, is a cylindrical molybdenum heatreflector 43. With dimples 49 formed in the cylindrical shield, it alsofunctions as a spacer interposed between the cathode cylinder 29 and aninner heat reflector 50 formed from molybdenum. The latter reflector iscoaxially arranged within t.e cylindrical reflector 43 and the cathodesupport cylinder 29 and is confined between the convex bottom surface ofthe cathode and the upper concave surface of the heater plate 44. Itwill thus be seen that the cathode and cathode heater assemblycomprising cathode support cylinder 29, cathode 31, heat reflectors 35,37, and 39, heat shield 42, heater coil, and molybdenum heater plate 4are rigidly interrelated with the coaxially arranged molybdenum pillaror spacer 46 and the cylindrical reflectors 48 and 51 Such a cooperativerelationship of elements lends itself to fabrication by mass productiontechniques, and results in an assembly which is inherently rigid.Additionally, the relationship of the elements is particularly adaptedto prevent the radiation of heat away from the cathode in both an axialand a transverse direction. Such conservation of heat makes possible useof lower power in the heater coil, and thus results in a saving in theoperation of the tube. It also lends itself to assembly as a compositeunit which may later be adcquately supported within the gun structure bybeing spot welded to the upper end of the cathode assembly supportcylinder 27.

To further ensure against the radiation of heat in a direction away fromthe cathode, another molybdenum heat shield 51 is integrally securedwithin the cathode assembly support cylinder 27 intermediate its ends.This shield is also preferably concave and is axially spaced from thereflector 36. A peripheral edge portion of the shield 51 is supported atcircumferentially spaced intervals on nickel tabs 52 spot welded to thecylindrical wall of the assembly support cylinder 27. Supporting theperipheral edge portion of the molybdenum heat shield at spacedintervals around the periphery lessens the conductive effect of the tabsand reduces the amount of heat that is dissipated by conduction into theassembly support cylinder 27 The tabs also function to-accomrnodatslight displacement between the'par t s due to thermal expansion andcontraction, and thus prevent distortion of the parts.

Because of its low vapor pressure the molybdenum shield 51 is capable ofwithstanding the high temperatures which it will encounter in itsproximity to the heater coil assembly. In this position it will functionto shield a further apertured heat shield 53 from the effects of thehigh temperature. The latter shield is preferably fabricated from copperto take advantage of its high coefflcient of heat reflectivity in theinfra-red spectrum, but because copper characteristically has a highervapor pressure, it is located in a relatively low temperature zone. Itis provided around its peripheral edge with a continuous support ringhaving one flange 54 brazed adjacent the peripheral edge of the coppershield and a cylindrical flange 56 brazed to the assembly supportcylinder 27. It will thus be seen that the copper and molybdenum shields51 and 53 function to support and rigidity and form the bottom of arecess in the assembly support cylinder 27 in the nature of a wellwithin which the previously assembled cathode-heater coil package issuspended.

Axially spaced below the cup shaped copper shield 53, and integrallybrazed adjacent the upper end portion of the cylinder 26, are aplurality of radially extending and axially spaced flat heat shields 57,spaced apart by peripheral flanges 58. This group of heat shieldsfunction to further isolate the cathode-heater package from relatedstructure to prevent the radiation of heat thereto. As shown in FIGURE2, apertures 61 in the heat shields, and apertures 62 in the cylinder26, provide for the escape of air molecules from within these chambersand prevent entrapment of air therein upon evacuation of the envelope.Aligned apertures 63 in the reflector 36 and shield 53 adjacent theouter peripheries thereof cooperate with appropriate apertures 63A inthe cylindrical support cylinder 29 to further ensure against theentrapment of air 3 molecules.

To connect the heater coil into an energizing circuit, the heater coilis provided with leads 64 and 65, both of which extend through thealigned apertures in the shields in spaced relation thereto, the lead 64being provided with a tab 66, one end of which is brazed or spot weldedto the lead 64, and its other end spot welded to the inner surface ofcylinder 27. Lead 65 is also provided with a connecting tab 67 connectedat one end to the lead and at the other end to the upper end portion ofa terminal rod 68. The tabs 66 and 67 are preferably fabricated fromtantalum, which is capable of withstanding high temperatures, while theterminal lead 68 is fabricated from tungsten, which is also capable ofwithstanding high temperatures. The lower end of the terminal rod 68 isbrazed to the upper end of a copper rod 69, the lower end of which isbrazed into a nickel plug '71. The plug is on the opposite side of thebase plate from the electrodes within the gun envelope, and constitutesan external terminal for the heater coil. The nickel plug is providedwith a radially extending flange 72 which abuts an annular plate 73formed from copper and which is integrally brazed to an annular coppersealing flange 74 at its outer periphery. The inner periphery of theannular sealing flange is integrally and hermetrically brazed to one endof a short ceramic cylinder 76. A second ceramic cylinder 77 in thenature of a backing ring is brazed adjacent the inner periphery of thesealing flange 74 on the opposite side thereof from the cylinder 76, andcooperates with a second annular sealing flange 78 brazed adjacent itsinner periphery to the opposite end of the ceramic cylinder 77. Theouter pe riphery of the annular sealing flange 78 is integrally brazedin a rabbeted groove formed in the edge of aperture 13 in the base plate2. A third short ceramic cylinder 79 is integrally brazed to sealingflange 78, and with the cylinders 76 and '77 rigidly supports theflanges, and seals the aperture 13 against atmospheric pressure. Anappropriate ly tapped bore 81 in the nickel plug 71 provides aconvenient means for connection of a lead to a source of electric power.From the foregoing it will be apparent that a very effective means hasbeen provided for insulatingly yet rigidly extending a conductor throughthe vacuum wall of the envelope represented by the base plate 2, and itwill also be obvious that materials having low vapor pressure and highcost have been reduced to a minimum and have been arranged closer to thesource of heat, and that the arrangement is such that other materialshaving somewhat higher vapor pressure but being lower in cost have beenutilized to a large extent.

Coaxially surrounding the cathode supporting sleeves 26 and 27 is afocus electrode supporting cylinder 82. The focusing electrodesupporting structure comprises a supporting flange fabricated preferablyfrom monel metal or stainless steel and includes a radially extendingflange 83 and an integral cylindrically extending flange 84. Thecylindrically extending flange 84 is spot welded to the lower endportion of the focus electrode cylinder 82, whhe the upper end of thecylinder 82 is flared in a portion 86, and spot welded to the peripheralsurface portion 87 of the focus electrode, designated generally by thenumeral 88. The focus electrode preferably has a truncated conical crosssection, and is provided at its upper end with a cylindrical section 89,to which is integrally brazed an arcing shield or ring 91, theconfiguration of which prevents arcing between the focus electrode andrelated structure. The lower edge 92 of the focus electrode ispositioned closely adjacent the outer peripheral portion of the cathodeand thus functions to form the beam of electrons emitted by the cathode.

As shown in FIGURE 2 the radially extending flange 83 of the focuselectrode supporting structure is sandwiched between annular dielectricwafers 93. The di electric wafers function to mechanically support thefocus electrode structure and to insulate it from the base plate. Screws94 extending through the central apertures of the annular ceramic wafersand threaded into posts 7 brazed to the base plate securely mount thefocus electrode on the base plate. It will thus be seen that the focuselectrode is detachably supported on the base plate, and is providedwith a rigid supporting base structure, to prevent the effects ofvibration. It will also be apparent that the arrangement of parts lendsitself to fabrication by assembly line techniques such as automaticmachinery and self-aligning or self jigging parts.

To electrically charge the focus electrode with an energizing potential,a terminal rod 96 extends through an aperture 13 in the base plate, andis hermetically supported and sealed thereto as was described withrespect to the rod 69. Like parts have been correspondingly numbered.integrally spot welded to the upper free end of the terminal rod 96 is atantalum strip or tab 97, the free end of which is integrally spotwelded to one end of a tantalum conductor 98. The other end of theconductor 98 is conductively connected to the focus electrode supportingstructure by being spot welded to the radially extending flange 83. Itwill thus be seen that by connecting the nickel plug 71 associated withthe terminal rod 96 to a source of electric energy the focus electrodemay be electrostatically charged in order to perform its function offocusing or forming the electron beam.

As shown in FIGURE 2, the free end of the ceramic wall section 14 isprovided with an integrally brazed sealing flange 99, adapted to behermetically brazed to the associated envelope structure. The associatedstructure includes a connecting annular copper plate 101 integrallybrazed to a cylindrical metallic envelope portion 102, preferably ofcopper and functioning as a corona shield, the upper end of which isintegrally brazed to an accelerating anode 103, which includes a shortdrift tube section 104. The accelerating anode 103 is integrally andhermetically interposed between the copper corona shield 102 and anassociated dielectfic envelope portion 106 by appropriate sealing means107.

From the foregoing it will be apparent that the elements of the electrongun are arranged in a relationship which is conductive of a maximumamount of cooperation between the various parts to efficientlyre-radiate heat towards the cathode, and which also prevents thedetrimental effects of vibration resulting from the particularapplication in which the gun is used. The cooperative combination alsotakes advantage of the cooperative relationships disclosed to permitutilization of less costly materials having higher vapor pressures inlocations which conventionally require more costly materials havinglower vapor pressures. By this means applicant has materially reducedthe cost of materials utilized in the electron gun, and by designing theindividual parts for facility of fabrication by automatic machinery, hasfurther achieved a saving in the cost of labor of fabricating theelectron gun. The electron gun may therefore be produced faster, at lesscost to the consumer, with a measure of reliability of operation notbefore attained in electron guns of the prior art.

I claim:

'1. In an electron tube having an evacuated envelope, an electron guncomprising a base plate constituting part of the evacuated envelope, ahollow metallic support cylinder detachably secured at one end to thebase plate and extending into the envelope, an electron emittingassembly including electrically energizable indirect heating meanssupported on the end of the support cylinder remote from the base plate,an accelerating anode within the envelope operatively arranged adjacentthe emitting surface of the cathode to attract electrons therefrom whenthe indirect heating means is energized, and heat reflector meanssuspended from the end of said cylinder remote from the base plate toreflect heat from said heating means toward the cathode.

2. In an electron tube having an evacuated envelope, an electron guncomprising a base plate constituting part of the evacuated envelope, ahollow metallic support cylinder detachably secured at one end to thebase plate and extending into the envelope, an electron emittingassembly including an apertured cathode having an emitting surface and anon-emitting surface, cathode heating means axially spaced from saidnon-emitting surface, a hollow receptacle closed at one end and havingan open end in registry with the aperture in the cathode, said electronemitting assembly being supported on the end of the support cylinderremote from the base plate, an accelerating anode within the envelopeoperatively arranged adjacent the emitting surface of the cathode, andheat reflector means associated with said cathode heating means toreflect heat toward the cathode.

3. In an electron tube having an evacuated envelope, an electron guncomprising a base plate constituting part of the evacuated envelope, ahollow metallic support cylinder detachably secured at one end to thebase plate and extending into the envelope, a cathode supported on theend of the support cylinder remote from the base plate and havingemitting and non-emitting surfaces, said non-emitting surface facinginto the support cylinder, an electrically energizable heater coilwithin the support cylinder axially spaced from the non-emitting surfaceof the cathode, means depending from said end of the support cylinderremote from the base plate and retaining the heater coil in said supportcylinder, means interposed between the coil and the cathode forretaining the heater in said spaced relation to the cathode, and ananode within the envelope operatively arranged adjacent the emittingsurface of said cathode to attract electrons therefrom when said heatercoil is energized.

4. In an electron tube having an evacuated envelope, an electron gunincluding an indirectly heatable cathode and heating means therefor, andheat reflector means associated with said cathode andheatifigmeans andincluding a cylindrical heat reflector arranged to reflect transverselyradiated heat toward the cathode and a dishshaped heat reflectorarranged to reflect axially radiated heat toward the cathode, saidcylindrical and dish-shaped heat reflectors defining with said cathode ahollow chamber enclosing said heating means, means radially spaced aboutsaid hollow chamber for supporting said cathode, heating means and heatreflector means on said evacuated envelope, and an electron acceleratinganode within the envelope operatively associated with said cathode.

5. A cathode-heater assembly for electron tubes comprising a cathodehaving a peripheral edge radially spaced about a central axis, acylindrical heat reflector integral at one end with the peripheral edgeof the cathode, a transversely extending reflector closing the other endof the cylindrical heat reflector, a cathode heater coil interposedbetween the cathode and the transversely extending reflector andenergizable to heat the cathode, and means interposed between the coiland the cathode and between the coil and the transverse reflector tosupport the coil and reflect heat therefrom toward the cathode.

6. A cathode-heater assembly for electron tubes comprising a cathodehaving a peripheral edge radially spaced about a central axis, acylindrical heat reflector integral at one end with the peripheral edgeof the cathode, a transversely extending reflector closing the other endof the cylindrical heat reflector, a cathode heater coil interposedbetween the cathode and the transversely extending reflector andenergizable to heat the cathode, cylindrical spacer means coax-iallyarranged within the cylindrical reflector and interposed between thecoil and the cathode to retain the coil axially spaced from the cathode,and spacer means interposed between the transverse reflector and thecoil to retain the coil axially spaced from the transverse reflector.

7. The combination according to claim 6, in which said cylindricalspacer means include a cylindrical shell coaxially arranged adjacentsaid cylindrical reflector and a cylindrical pillar coaxially arrangedadjacent the central axis of the assembly. 1

8. The combination according to claim 6, in which said spacer meansinterposed between the transverse reflector and the coil comprise atransversely extending dimpled spacer plate, a second plate parallel tosaid spacer plate and axially spaced therefrom by said dimples andhaving in tegral axially extending peripheral tabs thereon, and atransverse reflector plate supported on said tabs in spaced relation tosaid second plate and abutting the side of said coil remote from thecathode.

9. The combination according to claim 7, in which the cathode iscentrally apertured and the cylindrical pillar is hollow and is providedwith at least one open end registering with the aperture in the cathode.

10. The combination according to claim 7, in which said cathode isdish-shaped to provide a concave emissivc surface and a convexnon-emissive surface, and means are interposed between said cylindricalspacer means and the coil to efiect conformation of the coil with theconvex surface of the cathode.

11. In an electron tube having an evacuated envelope including axiallyaligned cylindrical dielectric and metallic portions, an electron guncomprising a base plate constituting part of the evacuated envelope andclosing one end of the dielectric envelope portion, a hollow metallicsupport cylinder detachably secured at one end to the base plate andextending into the envelope, an electron emitting assembly includingelectrically energizable indirect heating means supported on the end ofthe support cylinder remote from the base plate, an accelerating anodewithin the envelope supported on the cylindrical metallic portionthereof and operatively arranged adjacent the emitting surface of thecathode to attract electrons therefrom when the indirect heating meansis energized, and heat reflector means suspended from the end of saidcylinder remote from the base plate to reflect heat from said heatingmeans toward the cathode.

References Cited in the file of this patent UNITED STATES PATENTS2,441,224 Hector May 11, 1948 2,680,209 Veronda June 1, 1954 2,701,320Kovach Feb. 1, 1955 2,879,428 Williams Mar. 24, 1959 2,889,478 RogersJune 2, 1959

1. IN AN ELECTRON TUBE HAVING AN EVACUATED ENVELOPE, AN ELECTRON GUN COMPRISING A BASE PLATE CONSTITUTING PART OF THE EVACUATED ENVELOPE, A HOLLOW METALLIC SUPPORT CYLINDER DETACHABLY SECURED AT ONE END TO THE BASE PLATE AND EXTENDING INTO THE ENVELOPE, AN ELECTRON EMITTING ASSEMBLY INCLUDING ELEACTRICALLY ENERGIZABLE INDIRECT HEATING MEANS SUPPORTED ON THE END OF THE SUPPORT CYLINDER REMOTE FROM THE BASE PLATE, AN ACCELERATING ANODE WITHIN THE ENVELOPE OPERATIVELY ARRANGED ADJACENT THE EMITTING SURFACE OF THE CATHODE TO ATTRACT ELECTRONS THEREFROM WHEN THE INDIRECT HEATING MEANS IS ENERGIZED, AND HEAT REFLECTOR MEANS SUSPENDED FROM THE END OF SAID CYLINDER REMOTE FROM THE BASE PLATE TO REFLECT HEAT FROM SAID HEATING MEANS TOWARD THE CATHODE. 